Azobisisobutyronitrile (or AIBN) is an azo compound that is commonly used in radical polymerization processes, as an initiator or catalyst. It is also known as a swelling agent for the manufacture of PVC foams or silicone seals.
AIBN is conventionally produced by converting acetone cyanohydrins into hydrazine, followed by oxidation with gaseous chlorine (U.S. Pat. No. 2,469,358; WO 2006/067 315). This process has also been applied to other azo compounds such as azodicarbonamide (GB 976 552). It has the major drawback, besides the intrinsic dangerousness of chlorine, of generating hydrochloric acid as a by-product, and as such the effluents, which are incidentally produced in large amounts, cannot be readily recycled. It is understood that the process with chlorine is therefore unsuited to the current environmental constraints.
To overcome this drawback, processes have been proposed for synthesizing azo compounds not using chlorine, but hydrogen peroxide as oxidizing agent. These processes require the presence of an activating agent, generally a bromine compound such as a bromide, or even an iodine derivative, used in acidic medium, in order for the reaction to be sufficiently fast and complete. Such processes have especially been described in documents U.S. Pat. No. 4,637,868; RO 90707; RO 107 406; RO 106 881. The reactivity of hydrogen peroxide, and thus the reaction yield, are generally improved by using metal catalysts in addition to the bromine or iodine compounds (Drug et al., JACS 2007, 129(45), 13784-13785; JP52-144622; JP52-133924), in particular catalysts based on molybdenum or tungsten (CS 237 123; CS 239 407; CS 232 350; Palomo et al., Afinidad, 1985, 42(397), 312-314) which have the advantage of being less toxic than tellurium, vanadium or selenium, for example.
Despite its undisputed advantages relative to the process with chlorine, the process with hydrogen peroxide, however, also itself generates effluents that are potentially hazardous to the environment, namely the mother liquors resulting from the filtration of the reaction medium to separate out the AIBN. These mother liquors in fact contain an appreciable amount of the catalyst and of the activators used. It has thus been suggested, not only for environmental reasons, but also to improve the economy of the process, to recycle these mother liquors into the reaction, optionally after concentration (CS 239 407; CS 237 123).
Unfortunately, it has been observed that in the presence of certain catalysts, the mother liquors are unstable over time and have a tendency to precipitate out, which is detrimental to their recycling or to their processing for the purpose of discharging into the environment. The present inventors have demonstrated that the addition of certain reducing agents to the reaction medium makes it possible to solve this problem.
A process is thus proposed for synthesizing azo compounds of high purity, with a yield of greater than 90%, allowing easy treatment of the effluents for the purpose of recycling them into the process and/or of discharging them into the environment, such that the economy of the process and its environmental impact are improved.